1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Color image forming apparatuses using an electrophotographic method or an electrostatic recording method have been increasing. Various types of printers, copying machines, and facsimiles (FAXes) are on the market.
As a representative example, a rotary type image forming apparatus has been discussed. A plurality of developing devices containing toners of respective different colors is prepared for a single photosensitive drum (electrophotographic photosensitive member) serving as an image bearing member. The plurality of developing devices successively develops electrostatic latent images on the photosensitive drum. Specifically, a rotating developing device (referred to as a rotary or carousel) integrally including developing devices of four colors, e.g., yellow, magenta, cyan, and black, is arranged near a single photosensitive drum. Electrostatic latent images are formed on the common photosensitive drum. The electrostatic latent images are visualized as toner images in a development position where the developing devices are rotated to reach. A primary transfer unit transfers the toner images formed on the photosensitive drum to an intermediate transfer belt. The color toner images are successively and selectively superposed on one another to form a multicolor toner image on the intermediate transfer belt. The multicolor tone image is then transferred to a transfer material in a collective manner.
As another method, an inline color image forming apparatus has been discussed. The image forming apparatus of such a method includes a plurality of photosensitive drums serving as image bearing members. The photosensitive members are opposed to respective color developing devices, which separately form toner images of respective colors. Primary transfer units successively transfer the toner images from the respective photosensitive drums to a transfer belt to form a superposed toner image in four colors. A secondary transfer unit then transfers the toner image to a transfer material in a collective manner to form an image.
Inline color image forming apparatuses have recently been becoming mainstream because the inline color image forming apparatuses are more advantageous than rotary type color image apparatuses in terms of the productivity of color prints. However, since a plurality of photosensitive drums needs to be used to separately perform image formation, the inline color image forming apparatuses have the disadvantage of increased complexity. To deal with this disadvantage, Japanese Patent Application Laid-Open No. 2011-158676 discusses using a common power supply to apply high voltages to a plurality of primary transfer members.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2011-158676, the image forming apparatus can be simplified. However, if photosensitive drums having different degrees of wear are mounted on stations (image forming units), it is difficult to maintain both favorable transferability and retransferability in all the stations.
The reason is that a charged portion potential (Vd) and an exposed portion potential (VL) vary from one station to another depending on the degrees of wear of the photosensitive drums in the stations.
Transferability refers to the characteristic of moving toner (developer) from a photosensitive drum to an intermediate transfer belt (transfer member). The transferability depends mainly on a difference (transfer contrast) between the exposed portion potential and the potential of a primary transfer member. Retransferability refers to the characteristic that the toner transferred to the intermediate transfer belt returns to the photosensitive drum. The retransferability depends mainly on a difference (retransfer contrast) between the charged portion potential and the potential of the primary transfer member. The potential of the primary transfer member can be set to increase the rate of the developer moving from the photosensitive drum to the intermediate transfer belt and reduce the rate of the developer returning from the intermediate transfer belt to the photosensitive drum.
According to the configuration discussed in Japanese Patent Application Laid-Open No. 2011-158676, the potentials of the primary transfer members in the respective stations cannot be independently controlled. If the charged portion potential and the exposed portion potential of the photosensitive drums vary from one station to another, the retransfer contrast and the transfer contrast also vary station by station. In such a case, some of the stations may fail to maintain favorable transferability and/or retransferability.